The present invention relates to a method for improving an abnormal profile for example, T-topping and top rounding, of a chemically amplified resist due to influence of contaminants in the air.
Recently, lithography using a chemically amplified resist is adopted for forming a fine pattern of a semiconductor device. However, in the lithography using the chemically amplified resist, there is a serious problem that a predetermined pattern width can not be obtained because of an abnormal profile after development, for example, T-topping and top rounding. The abnormal profile of the chemically amplified resist is caused by the following reasons.
When a chemically amplified resist is applied to a semiconductor substrate and subjected to post-apply bake (PAB), and then exposed to light, an acid generates from an acid generator contained in the chemically amplified resist. However, if contaminants exist in the air, the acid concentration in the resist varies during post-exposure bake (PEB) conducted subsequently. As a result, when the resist is developed, a resist pattern having an abnormal profile is formed.
Namely, if basic contaminants such as an amine exist in the air, the acid concentration near the surface of the resist decreases due to a neutralization reaction. Since the exposed regions where the acid concentration has decreased are not easily dissolved in a developing solution, T-topping occurs.
On the other hand, if acidic contaminants exist in the air, the acid concentration near the surface of the resist increases. Since the exposed regions and their neighboring regions where the acid concentration has increased are easily dissolved in a developing solution, top rounding occurs.
The following method is conventionally known to improve an abnormal profile of a chemically amplified resist. For example, Hanawa (U.S. Pat. No. 5,429,910) discloses a method of treating the surface of the resist after exposed to light with a solution or vapor of sulfuric acid or carboxylic acid. However, if the surface of the resist is treated with acid, it is difficult to control the acid concentration in the exposed region of the resist. Therefore, this method is not regarded as an effective method for improving the resist profile after development.